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Having penned two teen novels in the late 1990s while she was still a teenager herself, Isla Fisher believes she would have become a novelist if her acting career hadn't proved so successful.

But after four years playing Shannon Reed in Home and Away, the Perth-raised actress made the transition to Hollywood and films like Scooby Doo, I Heart Huckabees, Wedding Crashers and fellow Australian Baz Luhrmann's lavish big-screen version of The Great Gatsby. Now the 40-year-old has come full circle, with her first book for children, Marge in Charge.

"When I was 18, my mum encouraged me to write those two books," she recalls, referring to Bewitched and Seduced by Fame, which were published in 1996 when Fisher was 20. "We had so much fun, until it got to collaborating on the love scenes!"

Born in Oman to Scottish parents, Fisher moved to Western Australia when she was 6 years old. Admitting her adopted home of Perth was an isolated place to live, she rode horses and led "a very outdoorsy life", but also developed a keen interest in literature.

Having met at a party in Sydney in 2002, Fisher married Ali G/Bruno creator Sasha Baron Cohen in 2010 and the couple now have three children, 8-year-old Olive, 5-year-old Eluha and 1-year-old Montgomery.

Dreamed up as a way of persuading them to go to sleep, the mischievous Marge has taken on an existence of her own with the publication of Marge in Charge. Citing Roald Dahl, Jeff Kinney, Rachel Renee and fellow actor-turned-author David Walliams, Fisher points out there are plenty of amusing books for the 7-to-10-year-olds but not so much for 4-to-6-year-olds.

"The funnier books, like Horrid Henry, have situations where the parents have no control over the characters' unruliness and name-calling, so I felt there was a hole in the market for that age group with some more innocent sweet humour," says Fisher, who has tailored the book to a younger age group's more limited reading abilities.

"Each book has three short stories inside, which suits the shorter concentration span of the age of the children they are aimed at."

Hired by the mother and father of Jemima and Jake Button to look after their children in their absence, Marge is an anarchic if eventually level-headed influence, initially causing chaos before restoring order just in time for the parents' return.

"Marge has to appear sensible in front of Mummy and Daddy or parents wouldn't understand why Jake and Jemima's mummy and daddy trust her with their kids. But then once they have gone, the fun begins," says Fisher.

"I find that kids are used to other kids misbehaving but when grown-ups do it, they find it hilarious. When Marge misbehaves, it's not in an intentional or mean-spirited way, as she unlocks the other characters' creativity and challenges Jemima's need to follow the rules for rules' sake."

With her rainbow-coloured hair belying her somewhat advanced years, Fisher suggests that Marge's senior status allows her to form a bond with Jemima and Jake.

"The novel is a celebration of generational relationships, as even though Marge is an old lady, she is more fun than other kids of Jake and Jemima's age, and she behaves like she is younger than their mummy and daddy."

Fisher would love to see Marge brought to life on screen but refuses to confirm whether she would play the gregarious babysitter herself in either a big or small-screen adaptation.

After playing a MI6 handler in Baron Cohen's latest film, Grimsby, earlier this year, she'll next be seen in Superbad director Greg Mottola's espionage comedy Keeping Up With the Joneses. "

I play a bored suburban housewife, who is married to Zach Galifianakis," says Fisher, who has also completed work on a dark thriller, Nocturnal Animals, Tom Ford's highly anticipated follow-up to the Oscar-winning A Single Man.